Summary of Work: The mucociliary epithelium of the conducting airways produces mucinous as well as non-mucinous secretions which protect the epithelium against toxic air contaminants, microbial agents and cytotoxic products released by inflammatory cells. However, mucous hypersecretion can be a serious complication in various airway diseases including cystic fibrosis, asthma and bronchitis. The objective of our studies is to elucidate mechanisms regulating airway secretions using cultured normal human tracheobronchial epithelial (NHTBE) cells. One series of studies is concerned with the role of retinoic acid (RA), a key metabolite of retinol (vitamin A), in the control of mucin gene expression and mucin secretion. We concentrated on two mucin genes, namely MUC2 and MUC5AC, because we suspect that they may be particularly important in the production of respiratory tract mucin. Using competitive RT-PCR to quantitate mucin mRNA levels we found that MUC2 and MUC5AC mRNA expression coincides with the morphological differentiation of RA- sufficient NHTBE cultures to a columnar, mucous epithelium and with the onset of mucin secretion (undifferentiated cultures do not express MUC2 or MUC5AC and do not secrete mucus). In RA-deprived cultures MUC2 mRNA was induced within 12 to 24 hrs and MUC5AC mRNA at 48 hrs following RA treatment, suggesting that different mechanisms may be involved in the regulation of these two mucins. Studies with retinoid receptor selective agonists indicated the importance of RARbeta and RARgamma in the induction of these two mucin genes. The role of RARbeta is still uncertain. A RXR selective agonist failed to induce mucin gene expression in RA-deficient cultures. Currently, studies are under way to identify regulatory sequences in the MUC2 promoter which are involved in the RA- dependent regulation of this mucin gene.